1. Field of the Invention
The present invention relates to a common-rail fuel-injection system in which fuel supplied under high pressure from a common rail is injected under pressure into the combustion chambers of engines.
2. Description of the Prior Art
The various types of fuel-injection systems for engines include a common-rail fuel-injection system in which the fuel stored under high-pressure in the common rail is applied to the injectors, which are in turn actuated by making use of a part of the high-pressure fuel as a working fluid to thereby spray the fuel applied from the common rail into the combustion chambers out of discharge orifices formed at the distal ends of the injectors.
Referring to FIG. 5 where an example of a conventional common-rail fuel-injection system is illustrated schematically, a fuel feed pump 6 draws fuel from a fuel tank 4 through a fuel filter 5 and forces it under a preselected intake pressure to a high-pressure, fuel-supply pump 8 through a fuel line 7. The high-pressure, fuel-supply pump 8 is, for example, a fuel-supply plunger pump driven by the engine, which subjects the fuel to a high pressure determined depending on the engine operating conditions, and supplies the pressurized fuel into the common rail 2 through another fuel line 9. The fuel, thus supplied, is stored in the common rail 2 at the preselected high pressure and forced to the injectors 1 through injection lines 3 from the common rail 2. The engine illustrated is a six-cylinder engine. The injectors 1 are arranged in combustion chambers, one to each chamber, of a multi-cylinder engine, for example, a six-cylinder engine in FIG. 5.
Excess fuel from the high-pressure, fuel-supply pump 8 is allowed to flow back to the fuel tank 4 through a fuel-return line 10. The unconsumed fuel remaining in each injector 1 out of the fuel fed from the common rail 2 into the injectors 1 may return to the fuel tank 4 through a fuel-recovery line 11. The controller unit 12 is supplied with various signals from sensors monitoring the engine operating conditions, such as a crankshaft position sensor for detecting the engine rpm Ne, an accelerator pedal sensor for detecting the depression Ac of an accelerator pedal, a high-pressure fuel temperature sensor and the like. In addition, the sensors for monitoring the engine operating conditions include an engine coolant temperature sensor, an intake manifold pressure sensor and the like. The controller unit 12 is also supplied with a detected signal as to fuel pressure in common-rail 2, which is transmitted from a pressure sensor 13 installed in the common rail 2.
The controller unit 12 may regulate the fuel injection characteristics of the injectors 1, including the injection timing and the quantity of fuel injected, depending on the applied signals, so as to operate the engine with the optimum injection timing and quantity of fuel injected per cycle in conformity with the present engine operating conditions, thereby allowing the engine to operate as fuel-efficiently as possible. The quantity of fuel injected per cycle is determined by the combination of injection duration with the injection pressure of the fuel sprayed out of the injectors. The injection pressure is substantially equal to the common rail pressure controlled by operating a flow-rate control valve 14, which is to regulate the quantity of high-pressure fuel delivered to the common rail 2. In case the injection of fuel out of the injectors 1 consumes the fuel in the common rail 2 or it is required to alter the quantity of fuel injected, the controller unit 12 actuates the fuel flow-rate control valve 14, which in turn regulates the quantity of fuel delivered from the high-pressure, fuel-supply pump 8 to the common rail 2 whereby the common rail pressure returns to the preselected fuel pressure.
Referring to FIG. 6, the injector 1 is comprised of an injector body 21, and an injection nozzle 22 mounted to the injector body 21 and formed therein with an axial bore 23 in which a needle value 24 is fitted for sliding movement. The high-pressure fuel applied to the individual injector 1 from the common rail 2 through the associated injection line 3 fills fuel passages 31, 32 and a fuel pocket 33 formed in the injector body 21. The high-pressure fuel further reaches around the needle valve 24 in the axial bore 23. Therefore, the instant the needle valve 24 is lifted to open discharge orifices 25 at the distal end of the injection nozzle 22, the fuel is injected out of the discharge orifices 25 into the combustion chamber. Provided at the distal end of the injection nozzle 22 is a fuel sac 26 to which are opened the discharge orifices 25. The needle valve 24 has a tapered end 27 that moves upwards off or downwards against a tapered surface 28 inside the injection nozzle 22 whereby the fuel injection starts or ceases.
The injector 1 is provided with a needle-valve lift mechanism of pressure-control chamber type in order to adjust the lift of the needle valve 24. The high-pressure fuel fed from the common rail 2 is partly admitted into a pressure-control chamber 40, which is formed inside the injector 1, past a fuel passage 35 branching away from the fuel passage 31 and a fuel passage 36 reduced in cross-sectioned area. The injector 1 has at the head section thereof a solenoid-operated valve 15, which constitutes an electronically-operated actuator to control the outflow of working fluid, or fuel from the pressure-control chamber 40. The controller unit 12 makes the solenoid-operated valve 15 energize in compliance with the engine operating conditions, thereby adjusting the hydraulic pressure of the working fluid in the pressure-control chamber 40 to either the high pressure of the admitted high-pressure fuel or a low pressure released partially in the pressure-control chamber 40. A control signal issued from the controller unit 12 is an exciting signal applied to a solenoid 38 of solenoid-operated valve 15.
The solenoid-operated valve 15 includes an armature 39 having at its end a valve body 42 for opening and closing an egress of a fuel leakage path 41. On energizing solenoid 38, the armature 39 rises to open valve body 42 whereby the fuel in the pressure-control chamber 40 is allowed to discharge, resulting in relieving the high pressure of the fuel in the pressure-control chamber 40. Although the valve body 42 is explained in the type of opening and closing the egress of the fuel leakage path 41, it may be alternatively made of a poppet valve composed of a valve stem extending through the fuel leakage path 41, and a tapered valve body provided at the end of the valve stem and having a valve face to make engagement with a valve seat at an ingress of the fuel leakage path 41.
A control piston 44 is arranged for axial linear movement in an axial recess 43 formed in the injector body 21 of the injector 1. Although the control piston 44 shown in the figure is formed integrally with the needle valve 24, the control piston may be formed separately from the needle valve and combined therewith such that they may be energized so as to follow one another. When the solenoid-operated valve 15 is energized to cause the fuel pressure inside the pressure-control chamber 40 to decrease, the consequent force, acting on the control piston 44 to push it downward, is made less than the fuel pressure acting on both a tapered surface 34 exposed to the pocket 33 and the distal end of the needle valve 24, whereby the control valve 44 moves upwards. As a result, the needle valve 24 lifts to allow the fuel to spray out of the discharge orifices 25. The quantity of fuel injected per cycle is defined dependent on the fuel pressure in the fuel passages and both the amount and duration of lift of the needle valve 24.
The common-rail fuel-injection system, or the pressure-balance, fuel-injection system, as described just above is disclosed in, for example, Japanese Patent Laid-Open Nos. 165858/1984 and 282164/1987, in in which the fuel supplied under pressure from the common rail 2 to the injectors 1 is partly applied to the chamber 40 in the injectors 1, acting as the working fluid to lift the needle valve 24 to thereby inject the fuel out of the discharge orifices 25.
It is well-known to those skilled in the art that the engine operating conditions in diesel engines are largely affected by the initial fuel-injection characteristics of the injectors 1, namely, the initial quantity of fuel injected, the initial injection rating and the rate of change thereof. For example, a large initial quantity of fuel injected causes a large quantity of fuel firing at the initiation of combustion with the heat release rate being increased whereby the diesel engines are apt to decline in noise control and exhaust gas performance. Not only the firing conditions at the initiation of combustion in the combustion chambers but the engine noise and the exhaust gas performance are affected by the time-base derivative of the initial quantity of fuel injected, or the initial injection rating, and the time-base rate of change of the injection rating. Nevertheless, no inexpensive, simple mechanism has been developed to determine how much the quantity of fuel injected, the injection rating and the rate of change thereof are at the actual initial fuel-injection. This causes such major problem that it is very hard to control reliably the injection rating and the like in most commercially available cars.
In recent years measuring means for the quantity of fuel injected out of the injector, as shown schematically in FIG. 7, has been developed, which is composed of a micro-turbine 50 arranged in a passage inside an inlet connector communicating the injection body 21 with the injection line 3 of the injector 1, and an optical sensor mechanism for detecting the rotational speed of the micro-turbine 50. Moving blades 51 are partially exposed in the fuel passage to thereby be turned by the fuel flowing through the fuel passage. Rotation of the moving blade 51 of the micro-turbine 50 block intermittently a light beam 52 from a light source to thereby output pulses of light, which are received at a detector. The peripheral velocity V of the moving blades 51 of the micro-turbine 50 at the blade tips is given by V=2 .pi. nR, where n is the rotational speed of the turbine, and R is the radius of the turbine. Because the peripheral velocity V is equal to the mean velocity of flow of the fluid, the flow rate of fuel may be obtained by measuring the rotational speed n over a preselected length of time. Moreover, the injection rating may be found by the differential of the flow rate of fuel with respect to time.
Nevertheless, the micro-turbine is of an extremely miniaturized turbine and, therefore, it is very hard to help ensure the accuracy in manufacture and the precision of measurement. Moreover, the optical sensor employed is inevitably expensive. In addition, the micro-turbine 50 disposed in the high-pressure fuel passages creates a flow resistance opposing the flow of fuel, resulting in probably affecting the fuel-injection characteristics.
Consequently, it is expected to develop the fuel-injection system in which the information as to the controlled variables of the fuel injection such as the quantity of fuel injected, the fuel-injection rating and the rate of change thereof at the early portion of the fuel injection may be obtained with the controller computing the detected results from the existing sensors for the engine operating conditions, with no need of additional measuring means, thereby feedback controlling the controlled variables of the fuel injection at the early portion of the fuel injection, or at the initial injection. Moreover, much attention has been given to the subject in which, even if the controlled variables of the fuel injection at the initial injection differ for each injector owing to differences in the fuel-injection characteristics for the individual injectors, the controlled variables of the fuel injection may be detected at every injector so that each of the injectors, may be subjected to the individual feedback control.